This invention relates generally to a continuously variable transmission (CVT) in which spaced pulleys are coupled by a tension-type belt. More particularly, it relates to an improved belt/pulley system which provides an increased ratio range resulting from an increase in the maximum operating pitch radius of the belt for a given maximum pulley cone radius.
The typical automotive transmission is shifted in finite steps between a high-torque, low-speed mode for starting a vehicle and a high-speed, low-torque mode for vehicle operation at highway speeds. In a manual transmission, shifting is accomplished by the selection and engagement of gear sets. In an automatic transmission, shifting is accomplished by the controlled engagement and release of friction elements. Because such shifting is in step functions, the most efficient vehicle operation can only be approximated. Efficiency would be improved by the provision of a transmission in which the drive ratio could be varied in a regular, continuous manner as the vehicle is started and accelerated to highway speeds.
It is known to use a CVT employing variable pulleys in machine tools and the like. Recently, considerable work has been directed to the application of a CVT to the drive train of an automotive vehicle. Examples of such CVT's are disclosed in U.S. Pat. No. 4,152,947 issued May 8, 1979 and U.S. Pat. No. 4,241,618 issued Dec. 30, 1980. These patents are incorporated herein by reference.
A CVT of this type may employ a tension-type belt which includes a plurality of struts guided by one or more endless bands. The struts transmit torque by compression. The bands guide the struts, and act in tension to retain the belt in its operating configuration. Examples of belts of this type are disclosed in U.S. Pat. No. 3,720,113 issued Mar. 13, 1972 and U.S. Pat. No. 4,080,841 issued Mar. 28, 1978. These patents are incorporated herein by reference.
By controlling the position of the pulleys, the operating pitch radius of an associated belt may be varied in a continuous manner without the step function changes noted above in connection with manual and automatic transmissions.
In an automotive environment in particular, space constraints often limit housing sizes, shaft center distances, pulley diameters, etc. Design parameters of this nature limit the ratio range of the belt/pulley system. Thus, there remains a need in the art to increase the ratio range of the belt/pulley system without increasing overall CVT size.